1. Nanjing University of Science &
Technology
A review of Paddle heat
exchangers
Presented by
Taha Hussein 塔哈
2. 1 • Introduction
2 • Applications of PHE
3 • Types of PHE
4 • Heat transfer analysis
5 • The summery
Presented by :
Taha Hussein 塔哈
The contents
3. Paddle heat exchanger(PHE):-
Paddle heat exchanger(PHE) is a kind of indirect heating
equipment with rotating surfaces that has been used in
many drying processes. When the tow shafts rotate
conversely, the outside material moves forward from
inlet end to outlet end driven by inclined rotating
paddles. Meanwhile, the material is heated by the hot
rotating surfaces which are heated by inside working
fluid.
In some applications, the outside convective
coefficient is in the same scale as that of the inside
working fluid, so to enhance working fluid heat transfer
is worthy for developing high performance paddle heat
exchangers. The construction , the different types, the
applications are the main points of the following PPT.
presented by :
Taha Hussein 塔哈
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
4. Main applications of paddle heat exchanger:-
1. Removing moisture :-
Paddle dryers, one kind of indirect dryers, are widely used in
removing moisture of sewage sludge ,brown coal ,coal slime
,etc. In this applications the working temperature is in the
range from 60 to 200oC.
2. Heating different types of materials in the industrial
applications:-
Paddle heat exchangers can be used for normal or vacuum
drying of soaked substances that are in the form of dry
powder mixtures or granulates in a manufacturing process
and are soaked and mixed (normally with solvents) due to
technological needs. In this applications the working
temperature is in the range from 200 to 600oC.
presented by :
Taha Hussein 塔哈
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
5. working substances in paddle heat exchangers:-
Normal working fluids include water , steam and heat
transfer oil in the temperature range less than 300oC.
In the temperature range 300 to 600oC, due to its
excellent heat transfer performance , molten salt (e.g. Hetic
salt) becomes favourable in diverse engineering
applications , such as in solar thermal power plants.
Generally, in these applications and as the previous
researches suggested the more sufficient working fluid in
this case of PHE applications is the Hitec salt (that is 53
%KNO3 – 40 %NaNO2 – 7 % NaNO3 based on mass
fraction).
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
8. A. Solid paddle heat exchangers .
B. Closed Hollow paddle heat exchangers.
C. Open hollow paddle heat exchangers.
Types of paddles:-
depending on the type of paddles we can classify the paddle heat exchangers to :-
presented by :
Taha Hussein 塔哈
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
9. Solid and hollow paddles:-
When we compare the heat transfer
rates for the three structures. In the range
Re = 26000 ~ 260000, the heat transfer
rate of the closed hollow paddle-shaft
structure is 3%~10% higher than that of
the solid paddle-shaft structure mainly
due to the air natural convection in the
closed cavities in paddles.
presented by :
Taha Hussein 塔哈
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
10. Open hollow paddles : -
The computational domain of one open hollow
paddle is shown in this Figure .In the left paddle
(Fig .a), the working fluid flows from wide end to
narrow end while in the right paddle, it flows
from narrow end to wide end. The open hollow
paddle-shaft structure transfers approximate 2.5
times heat of the solid paddle-shaft
or closed hollow paddle-shaft structures.
presented by :
Taha Hussein 塔哈
(α = 3±, L1 = 28 mm, L2 = 18 mm, L3 =
8 mm, L4 = 17 mm, t1 = 2 mm, t2 = 2
mm and L = 60 mm).
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
11. The temperature field of solid is shown in
this Figure. The results are for the
outside surfaces of the entire paddle-
shaft structures. The outside surface
temperature of open hollow paddles is
higher than that of solid paddles and
closed hollow paddles. For open hollow
paddles, in the corner regions far from
the open holes, the temperature
(equivalent to the local heat transfer
rate) is lower than that in the centre
region.
presented by :
Taha Hussein 塔哈
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
12. presented by :
Taha Hussein 塔哈
Steady Heat Transfer:-
In Fig..a, with the left paddle as an example, it is easy to
find that in the corners of the hollow paddle, there exist
two low-speed flow zones (or poor flow zones), meaning
that the working fluid does not bath the entire space
sufficiently or uniformly. So the heat transfer in the poor
flow zones is weak. The paddle area is actually not fully
utilized When the paddle height H (i. e., the hollow fin
height) increases, the poor flow zones will enlarge. One
way to enhance the working fluid heat transfer is to
eliminate or narrow the poor flow zones. And we can
solve this problem by using the guide plates as shown in
the figure ,so that we can enhance the heat transfer rate
by narrow the poor flow zones.
a)Poor flow zones. b) Cross section of paddle with guide plates (b = 6.5 mm)[1].
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
14. 01
Typical Configuration
The heater is composed of some static hollow
discs with central holes.
Working Process
Material is fed from the top of the
heater and pushed by the rotating
blades.
Material drops from upper disc to lower
disc forms material flow.
In the hollow discs, hot working fluid
flows from inlet to outlet and transfers
heat to the material.
Molten salt
In the
temperature
range 300℃-
600℃, molten
salt becomes
favourable.
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
15. There are two common types of inlet/outlet arrangements.
Side surface-positioned and bottom surface-positioned.
Paddles types
Subject investigated
Steady simulation:
Inlet/outlet arrangements;
Circumferential fins;
Guiding plates;
(flow and heat transfer of molten
salt and entropy generation rate
of the discs)
Dynamic simulation:
Cold-start process.
(temperature record)
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
16. 03
Effects of inlet/outlet arrangements
The heat transfer coefficient of B2 is obviously greater than that of S;
When the mass flow rate is low, the pressure drop gap between B2 and S is tiny. When the mass
flow rate is large ( >150), the pressure drop of B2 is larger than that of S.
The entropy generation rate of B2 is less than that of S, which indicates B2 is better than S from the
view of irreversibility minimization.
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
17. Velocity and
temperature fields
It can be seen that more
space is bathed by the
molten salt in B2 than in
S.
Side surface-positioned
design is usually
constrained by the height
of the hollow disc.
Bottom surface-
positioned design can
distribute the inlet flow
uniformly.
M%=128.77
18. Effects of inlet/outlet volume
When the inlet/outlet volume
increases, the pressure drop
remarkably decreases.
This also brings about the
decreasing of the entropy
generation rate.
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
19. Effects of fin number
With the increasing in n, the heat
transfer coefficient increases and
the pressure drop decreases.
The entropy generation rate
change is quite limited.
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
21. Effects of fin pitch
The effects of r on both flow and
heat transfer is not obvious for the
specified mass flow rate. When r =
0.125, the heat transfer coefficient
and the entropy generation rate is
a little bit higher.
1
2 1
D D
r
D D
23. Effects of guiding plate
Both the heat transfer coefficient and
pressure drop of B2G with guiding
plates are greater than that of B2.
Entropy generation rates of B2G and B2
are nearly the same but larger than that
of SG.
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
25. Dynamic behaviour in the vertical paddle
heat exchangers
The dynamic processes of paddle heat exchangers
include two types, i. e., cold-start process from
ambient temperature and working condition
transition process. The later dynamic process is
normal. However, for molten salt paddle exchangers,
one critical problem is that when the salt
temperature is lower than its melting temperature
Tmelting (142oC for the working fluid), it will solidify. So
in cold-start process, pre-heating is necessary. This
Figure shows the scheme of a molten salt paddle
heat exchanger experimental system based on hot
air pre-heating device built by the researchers in the
school of energy and power engineering (NJUST).
The hot air heated by electricity is used to heat the
paddle heat exchanger.
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
26. cold-start process in the horizontal
paddle
heat exchangers
The pre-heating time is about 390s for the
design B2G with n = 3 and 720s for the
design B2 with n = 0 respectively. The
result shows that the enhanced heat
transfer design also shortens the cold-start
time efficiently.
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
27. presented by :
Taha Hussein 塔哈
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
Rotary joints:-
To decrease the thermal
expansion(or temperature)
in the sealing packing
domain, we can insert heat
insulation layer in the
inner tube wall as shown
in this figure.(b). This
structure weakens the heat
flow from the hot working
fluid to the sealing
packing domain and more
heat is dissipated through
paths where there is no
heat insulation.
28. The summery:-
1- In the vertical PHE:
In this ppt the author derived the main parameters of the design method document different working fluids
such as molten salt, and heat transfer performance in paddle heat exchangers. The different applications of
paddle heat exchanger were discussed. the author clearly derived The main difficulties in the modern
designs which make it easy for the new researchers to follow up the previous work in this field. The
development in the design to enhance the heat transfer rate, and the previous researches with their results in
this field were clarify in this paper.
For many applications, the materials to be dried or heated are low-valued or even wastes, so the drying
energy consumption becomes the critical issue. One way to reduce the energy consumption is to introduce
solar-energy system assisted the paddle dryer system. So the author suggest for future research to make a
study in the design and the analyses of performance of this combination system , especially in the range of
300o ~ 600o C as a working temperature of the system.
2- In the horizontal PHE:
The bottom surface-positioned inlet/outlet structure is better than side surface-positioned structure from the
heat transfer view and the entropy generation rate view. Larger inlet/outlet volume helps to decrease pressure
drop.
Case study shows increasing fin number can possibly improve flow and heat transfer performance
simultaneously. The inlet/outlet guiding plates increase the heat transfer coefficient and the pressure drop at
the same time.
Introduction
Applications of
PHE
Types of PHE
Heat transfer
analysis
The
summery
